310 PERSONAL PREPARATION FOR RESEARCH. 



minute differences of time ; the mirror, for magnifying 

 the signs of minute movements, as in the needles of mag- 

 netometers and galvanometers ; the electric condenser, for 

 multiplying the effect of minute electric charges ; and so on. 



Experiments are usually arranged upon a suitable 

 degree of magnitude, such that the desired effect may be 

 conspicuous, and interfering circumstances as small as 

 possible, for the latter are often greater than the expected 

 result. An ordinary chemical analysis, for example, is 

 usually made upon from 20 to 100 grains of the sub- 

 stance, in order to keep within a moderate compass the 

 unavoidable errors of the process. When the expected 

 effect is small, the substance or apparatus for producing it 

 should be large, and the means of detecting it should be 

 very delicate. 



Experiments are, in some cases, made for the purpose 

 of discovering a new phenomenon, and in others for in- 

 vestigating an already known truth. In the latter in- 

 stance, if the phenomenon appears to be of an altogether 

 novel kind, the experiments have at the outset to be made 

 in a less systematic way, until some guiding idea of its 

 probable nature is obtained ; but, even in such a case, the 

 employment of a table or series of leading ideas for raising 

 hypotheses is of advantage. 1 In a series of experiments, 

 especially in those made for the purpose of determining 

 causes, we usually vary only one circumstance or condition 

 at a time, and draw from it a single or small number of 

 conclusions. In other cases, however, by means of a 

 single experiment we are sometimes enabled to make a 

 whole series of determinations at once. 



All experiments have limits of time ; some, however, 

 require very long periods. Sir William Thomson has in 

 progress some experiments of diffusion of an aqueous solu- 

 1 See p. 370. 



